This invention relates to refining elements for a refining disc for disc-refiners intended for the manufacture and/or treatment of fibrous pulps, where the refining element is produced by casting a steel alloy and hardened and heat treated to a hardness of at least 55 HRC.
Disc-refiners for the refining of lignocellulosic material, i.e. for the mechanical manufacture or treatment of so-called mechanical pulp, are known, for example, through Swedish Patent Nos. 506,822 and 402,019. These disk-refiners comprise two circular refining discs, which are rotated relative to each other, and have refining surfaces built-up of refining elements (normally called refining segments), which comprise bars and grooves and guide the pulp from the center out to the periphery during the refining operation. The refining surfaces are subjected to heavy abrasive wear due to hard foreign particles, such as sand, in the chips. In addition, the temperature is also high, often about 220xc2x0 C., and the wood yields an acid pulp with a pH, which during the manufacture of newsprint is about 6.5, but which during board manufacture is as low as 4-5, which requires corrosion resistance. In order to reduce the wear, alloys with precipitated carbides are used.
One of the objects of the present invention is to thus provide refining elements of the aforedescribed kind, which have an improved service life.
In accordance with the present invention, these and other objects have now been realized by the invention of a refining element for use in a refining disk in disk refiners for the treatment of fibrous pulp material, the refining element comprising a heat-treated cast steel alloy having a hardness of at least 55 HRC and comprising from about 2.7 to 3.2 percent by weight C, from about 0.5 to 1.0 percent by weight Si, from about 0.7 to 1.2 percent by weight Mn, from about 21.0 to 26.0 percent by weight Cr, from about 3.0 to 6.0 percent by weight V, up to about 0.5 percent by weight Ni, and up to about 0.5 percent by weight Mo. In a preferred embodiment of the refining element of the present invention, the remaining composition of the refining element comprises iron and impurities.
In another embodiment of the refining element of the present invention, the refining element comprises from about 2.8 to 3.1 percent by weight of the C.
In another embodiment of the refining element of the present invention, the refining element comprises from about 0.7 to 1.0 percent by weight of the Si.
In another embodiment of the refining element of the present invention, the refining element of comprises from about 22.0 to 25.0 percent by weight of the Cr, and preferably from about 23.0 to 24.5 percent by weight of the Cr.
Vanadium is a very strong carbide former with a considerably greater affinity to carbon than chromium and the vanadium carbide has a hardness, which clearly exceeds that of the chromium carbide. Already the solidification, precipitation of vanadium carbides is obtained which improves both the wear resistance abrasive wear, the corrosion resistance. The corrosion resistance is understood and explained in that every percent of vanadium binds up to 0.23% carbon. As a result, carbon content in the matrix decreases to a corresponding extent, which has the consequence that the carbon content available for chromium for the formation of carbides becomes lower. The chromium carbides, which are precipitated, contain also a certain amount of vanadium. Therefore, the proportion of chromium substitution dissolved in the matrix, which improves the corrosion resistance, increases. Due to the adapted contents of carbon, chromium and vanadium the primary precipitated carbides assume a desired size, so that the tenacity is not reduced thereby such that the primary precipitated carbides are too great. The fracture surfaces of the alloys according to the present invention are considerably more fine-grained than of the other chromium-alloyed casting alloys for refining elements. A material is thus obtained which has both improved resistance to abrasive wear and improved corrosion resistance. This is particularly important for refining segments, which are intended to be used for the refining of board pulp.
In order to achieve sufficient hardness of above 55 HRC, and preferably from about 57 to 63 HRC, after hardening and heat treatment, the hardenability must be sufficient. Therefore, the carbon content must be kept high, and if the stated analysis interval for the carbide forming elements Cr, Mo and V and for Ni are exceeded, the hardness will not be achieved. If the carbon content exceeds the stated analysis interval, the carbides grow and embrittle the material.
If the lower limits for Cr and V are not reached, the desired mixture of carbide types essential for the wear resistance is not obtained.
In order to achieve maximum wear resistance and tenacity, the material must be hardened and annealed in a conventional manner. In connection with this heat treatment a secondary carbide fraction is precipitated, which is more finely dispersed than the one obtained solidification.